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Human Anatomy

Chapter 3 : Embryology

Embryology

 The study of the developmental events that occur during the prenatal period

Embryology

 Begins with Fertilization:  A single fertilized cell divides by mitosis to produce all of the cells in the body.

The Prenatal Period

 The first 38 weeks of human development  between fertilization and birth.  The pre-embryonic period:  first 2 weeks of development  zygote becomes a spherical, multicellular structure.  The embryonic period:  third through eighth weeks  all major organ systems appear.

The Prenatal Period

 The Fetal Period:  Includes the remaining weeks of development prior to birth  The fetus continues to grow  Its organs increase in complexity

The Stages of Embryogenesis

 Cleavage:  zygote divides by mitosis  forms a multicellular structure called a blastocyst.  Gastrulation:  blastocyst cells form three primary germ layers  basic cellular structures from which all body tissues develop.  Organogenesis:  three primary germ layers arrange themselves in ways that give rise to all the organs within the body.

Gametogenesis

 Following birth, an individual undergoes maturation.  the body grows and develops  the sex organs become mature  the sex organs then begin to produce gametes

Chromosomes

 Human somatic cells contain 23 pairs of chromosomes for a total of 46.  22 pairs of autosomes  one pair of sex chromosomes.  Autosomes contain genetic information for most human characteristics.  Homologous chromosomes:  pair of similar autosomes

Diploid Cells

 A cell is said to be diploid if it contains 23 pairs of chromosomes.  2N = 46

The Sex Chromosomes

 The pair of sex chromosomes determines whether an individual is female (XX) or male (XY).  One member of each pair of chromosomes is inherited from each parent.

Gametogenesis

 Begins with meiosis.  Produces secondary oocytes in the female.  Produces sperm in the male.

Meiosis

 A type of cell division  Starts with a diploid parent cell  Produces haploid daughter cells (sperm or eggs/ova).

Meiosis I

 Meiosis results in the formation of gametes (sex cells).  In meiosis I:  homologous chromosomes are separated after synapsis  crossing over occurs.  In meiosis II:  sister chromatids are separated  sequence of phases resembles mitosis.

Prophase I

 Homologous, double-stranded chromosomes in the parent cell form pairs (synapsis).  Tetrad:  Pair of homologous chromosomes  Crossing over  occurs between the maternal and paternal chromosomes.

Metaphase I

 Homologous pairs of chromosomes line up above and along the equator of the cell.  Forms a double line of chromosomes.  Alignment is random with respect to maternal or paternal origin.

Anaphase I

 Pairs of homologous chromosomes separate and are pulled to the opposite ends of the cell.

Telophase I and Cytokinesis

 Nuclear division finishes  The nuclear envelopes re-forms  The cytoplasm divides  Two new haploid cells are produced

Prophase II

 Resembles the prophase stage of mitosis.  In each of the two new cells:  nuclear membrane breaks down  chromosomes collect together.  Crossing over does not occur in this phase.

Metaphase II

 The double-stranded chromosomes form a single line in the middle of the cell.  Spindle fibers extend from the centrioles at the poles to the centromere of each double-stranded chromosome.

Anaphase II

 The sister chromatids of each double- stranded chromosome are pulled apart at the centromere.  Each chromatid (single strand) is pulled to the opposite pole of the cell.

Telophase II and Cytokinesis

 The single-stranded chromosomes arrive at opposite ends of the cell.  A cleavage furrow forms  Cytoplasm in both cells divides  Produces a total of four haploid daughter cells.  These daughter cells mature:  sperm in males  oocytes in females.

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